Saturday, 15 July 2017

The Forgotten Crossbow

By Brother Ian who lives down on the South coast was up this way to buy a banjo (a Gibson Earl Scruggs signature model). He dropped in to visit to look at the crossbow I'm building and have a good talk about trigger mechanisms and stuff. He brought with him a crossbow that I'd made him as a Christmas present over 45 years ago, he wanted to give it back to me as he didn't really have room to store it nicely and he no longer used it. He was a bit worried that I'd be somehow insulted, but felt it would have a better home in my "collection".
To be honest I'd forgotten how good it was and I'd conflated that bow with some of my less successful early attempts. It was joy to see this one again as it has oodles of style with a sort of retro Art Nouveau target crossbow look.
It's only a small bow, basically a scaled up bow pistol with a stock.

My Brother filled me in with the story of its use... at that time he was working for the Natural History Museum in workshops out in Cricklewood. In the heat of summer they'd open the windows high up in workshop but feral pigeons would get in making a mess everywhere and with the very high ceilings there was no easy way to get them out, unless of course you had a light weight accurate crossbow! He says he must have shot more than a dozen easily, but the bolts didn't last long rattling around indoors, so he made blunt ones out of aluminium tube weighted at the and with round head nails (flat end outermost).

Feeling it in my hand again was a joy as the grip had been sculpted perfectly to fit my hand, the Aluminium Alloy prod is about 22" long and the whole bow only 24" long, draw weight is about 45-50# but it's perfect for close range target (or pigeon) work.
The biggest surprise was the trigger mechanism, I'd forgotten that I'd made one of that type and the advantage of it was soon apparent. The string slips down into a groove when cocked, but the groove is slightly angled such that the sting would just slip back out again if not held down by the catch closing above it. The advantage of this is that the upward force on the string as it tries to slip out is only a tiny fraction of the draw weight, so the trigger mechanism isn't subject to the great force that it would normally be and can thus be a more subtle and delicate affair with a nice light pull.
In case that has confused anyone, imagine the slot is sloped at 45 degrees and the pull on the string is 100#, that 45 degrees slope would direct equal amounts of force into the stock and upwards trying to slip the string up and out. If the slot is completely vertical, all the force in into the slot and the string won't pop out on it's own. So you see the angle of the slot controls how much force is on the mechanism. There is always a downside though, and in this case it is that the string isn't constantly touching the bolt like with the more conventional trigger mechanism.
The next day I made a new string and my Son and I had some fun shooting into the garage, although the aperture in the rear sight was a little small for shooting into the relative darkness of the garage. I opened it up by about 0.2mm and this helped, along with the additon of some extra illumination of the target. This opens up a whole area of investigation as there are many types of sights, V U, aperture etc all with pros and cons.
I've also shot it through the chrono and it gave a respectable 165 fps, a lighter bolt would gain some speed if needed, but it's only really meant for short range target work.

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